1. Field of the Invention
The present invention relates generally to methods and apparatus for coupling optical signals, and more particularly planar lightwave circuit (PLC) couplers with improved wavelength and polarization sensitivity.
2. Description of Related Art
Optical couplers are employed in a variety of planar lightwave circuits (PLCs), including, for example, Mach-Zehnder interferometer (MZI) based switches, MZI based filters, taps, etc. The lowest-loss 2×2 coupler is a directional coupler (shown in FIG. 1(a), also called an evanescent coupler), which includes two waveguides arranged in close proximity to allow coupling of light between the waveguides. The splitting ratio of the conventional directional coupler, however, is highly sensitive to: 1) the wavelength of the light propagated through the coupler, 2) the polarization of such light, and 3) variations in the PLC coupler structure (e.g. waveguide width) which can occur during fabrication (hereinafter collectively referred to as wavelength, polarization, and fabrication (WPF) splitting sensitivity).
There are three main approaches to decrease the WPF splitting sensitivity of 2×2 couplers: 1) couple to extra modes (i.e., couple the two input modes to many modes inside the coupler and then couple back to two output modes), 2) make the coupler asymmetric, or 3) use multiple sections.
A multi-mode interference (MMI) coupler (shown in FIG. 1(b)) is one example of a device for coupling to extra modes. This coupler excites more than two modes in the coupling region. The increased number of modes can decrease the splitting sensitivity to WPF changes because uncertainties are averaged out among many modes. However, MMI couplers typically exhibit undesirable excess loss, and are sensitive to waveguide-width variations that occur during PLC fabrication. Another example of a multimode coupler is the star coupler (shown in FIG. 1(c)). However, as with the MMI coupler, 2×2 star couplers typically exhibit high excess loss.
An asymmetric coupler (shown in FIG. 1(d)) is a directional coupler having two coupled waveguides of different widths. In directional couplers, the splitting ratio varies sinusoidally between 0% and some value (R) as the coupler length is varied (see FIG. 4). In a symmetric coupler, R is 100%. In an asymmetric coupler, R is <100% (since the asymmetric nature causes already coupled light to become out of phase with newly in-coupled light before 100% of the light can be coupled over as the coupler length is increased). Asymmetric couplers are typically designed such that R is a desired splitting ratio, and the coupler length is selected such that the splitting ratio achieved is near the peak of the sinusoid (see FIG. 4). Thus, the first-order dependence of the coupling ratio on the length is removed. Asymmetric couplers, however, are highly sensitive to variations of the waveguide width from a desired width, which can occur during PLC fabrication.
Another example of an asymmetric coupler is the “adiabatic coupler” (shown in FIG. 1(e)). The waveguides of the adiabatic coupler are asymmetric at one end of the coupling region and symmetric at the other end. The operation principle for these couplers is that the couplers vary from symmetric to asymmetric so gradually that each of the two eigenmodes of the local waveguide structure at the symmetric end convert perfectly to each of the two eigenmodes of the local waveguide structure at the asymmetric end. Adiabatic couplers, however, are comparatively very long thus increasing device size and cost.
Finally, multi-section couplers (e.g. the two section coupler shown in FIG. 1(f)) include two or more directional couplers connected with waveguides having a small path-length difference. The small path-length differences are chosen such that the WPF-dependences (or sensitivities) of the directional couplers partially cancel each other out. Two-coupler, three-coupler, and four-coupler multi-section couplers have been proposed and demonstrated. These multi-section couplers, however, are very long thus increasing device size and cost.